Signaling system including adjustable wave filter



March 29, 1932. c H' FETTER 1,851,091

SIGNALING SYSTEM INCLUDING ADJUSTABLE WAVE FILTER Filed Dec. 29, 1927 INVEN TOR.

6f EFezter BY M ATTORNEY CHARLES H. IE ETTEB, OF MILLBURN, NEW JERSEY, 'ASSIGNOR TO AMERICAN TELE- Patented Mar. 29, 1932 UNITED STATES PAT N IFF CB I PHONE AND TELEGRAPH COMPANY, A CORPORA'I IO1\ T'OE NEW. YORK SIGIN'ALINGSYSTEM INCLUDING ADJUSTABLE wavn FILTER Application filed December 29, 1927. Serial no. 243,388. f i

This invention relates'to signaling systems, and more particularly to signaling systems 7 employing piezo-ele'ctric crystalsarranged so that one band of signals-may be received to the exclusion of all signals lying outsidethe band.

This invention is also related to electrical wave filters adapted to transmit, with small or negligibleattenuation, currents of allfre quencies lying within a desired, particular range of frequencies, while attenuating and substantially suppressing currents of all frequencies lying outside of the limits of the desired, particular range. f c 1 This invention may find expression in an electrical wave filter involving a plurality of piezo-electric structures having a plurality of impedance elements so arrangedthat anyone of these'elements, or a plurality of these" elements of the same magnitudes, may be connected in series with a line, and a plurality of impedance elementsany one of which, or several of which having the same magnitudes, may be connected in shunt acrossthe line, the values of the series impedance elementsbeing so proportioned with respect to the values of theshunt impedance elements, thatthe arrangement will transmit, with small or negli: gible attenuation, currents of allfrequencies lying within a'desired, predetermined band while attenuating and substantially suppressing currents of-all'frequencies lying outside that band. I l f r g A's iswell-known in the art, a simple electrical wave'filter may consist of an electrical circuit network which has a negligible transmission loss over one range, while the loss may be appreciable and may become very high over different ranges. v If it be attempted "to design an electrical wave filter for high-im'- pedance'circuits with the elementsheretofore employed therefor, it will be found that the inductances required become impracticably' large and'that the capacities become very' small,;especially when the frequencies in-' volved are high. It'is' an-established fact that itjis exceedingly difficult to wind, a large inductance 'which will have an inductive reactance' overja wide range of frequencies, due

to the distributive capacities of theinducf 7 electrical wave filter by pie zo-electric. struc-Q its resonant frequency. AtfrequenciesjsQme-Q tance, Furthermorathe electricalresistance of practicable coils havinghigh inductance tends to greatly reduce the sharpness of-the cut-off obtainable in thesimple electrical wave filter, and therefore introduce another Eli serious difficulty. These dilficulties,as well 7 as others not mentioned, may in. part be avoided by the introduction of step-up and step-down transformers, in order that-the Y or if a wide frequency range is to be covered, for the same reasons as stated hereinabove;

In order to overcome these difficulties,.it proposed herein to replace some orallof the circuit elements of the simple and well-known tures, or bya plurality ofsimilar pieZo-electricstructures. Such a replacement maybe come apparent from the partial equivalence of the piezoelectric crystal to some form of electrical network consisting of suitable re actance elementsij That this partial equiva lence exists, will readily be apparentifa piezo-electric crystal be connected in parallel relationship with a tuned (circuit of? series inductance and capacity and the arrangement be fed with current'from a source ofvariable frequency. If the current in the tuned'fcir cuit mayTbe read on at meter, and the fre j quency of the source is .varied, it 'willbe found that the current therethrough reaches a maxi mum value at the frequency at resonance'of V the Ytuned circuit." Ifthe tuned circuit be so adjusted that its resonant frequency lies very close x to the resonant frequency of natural vibration ofthepiezo-electric crystal, andjif the current in this tuned circuit beplott ed asa function of frequency,.it will be foundthat.

, at the natural vibratory period of th'epiezo-o electric crystal the current in the tuned cirf caitwin drop to a very low value, rising as sharply thereafter as the resonant frequency, of'the crystal is past; In other words, the crystal acts insome respects like aseries resoe nant circuit of exceedingly low decrement at What "above the frequency "whichcurrentp through the crystal is a maximum, the piezoelectric crystal acts like an anti-resonant circuit. Moreover, the crystal will be found to include the equivalent of a resistance element representing dissipation due to its viscosity.

It is an object of this invention to provide an improved form of electrical wave filter which may comprise a plurality of recurrent sections, each section including a plurality of piezo-electric'devices and a plurality of corresponding reactive elements, whereby a desired piezo-electric device in each section, and a corresponding reactiveelement, may be selected in order to permit the free transmission of currents having frequencieslying within a band 'ofpredetermined limits, currents of all other frequencies being sub-stantially suppressed. It will be apparent that with a suitable selection of piezo-electric devices in' each recurrent section, the electrical wave filtermay be adjusted to freely transmit currents of any one ofaplurality of bands of frequencies.

It is another object of this invention to provide a signaling system capable of receiving a plurality of bands of frequencies with which piezo-electric means, or a piezo-electric wave filter of adjustable characteristic, may be associated for freely transmitting only one desired band of frequencies, currents of all other frequencies becoming greatly suppressed.

And it is another object of this invention to incorporate in a receiving system for receiving any one of a'plurality of bands of frequencies, each lying withina definiteposition of the spectrum, a pluralityof sets of piezoelectric devices, the various sets being equally graded and having substantially the same frequencies, one piezo-electric device in one a set corresponding to a similarpiezo-electric device in each of the other sets, a particular plezo-electrlc device 111 each of the various sets being selectedand interconnected in order 3 that currents of frequencies lying within a.

particular, predetermined band may become freely transmitted. It will be apparent that this invention embodies-a novel method for changing the receptivity from oneband of 3 frequencies to another band of frequencies,

the transmission characteristic of the interconnected circuit being considerably flatter within the selected transmitting band than has heretofore been possible with electrical wave'filters, and its attenuation characteristic being considerably steeper.

VVhile this invention will be pointed out with particularity in the appended claims, the invention itself, both as to its further objects and features, will be better understood from the detailed description hereinafter followingfwhen read in connection with the accompanying drawings, in which Figure 1 shows how an electrical wave filter, including piezo-electr-ic elements, may be embodied in transmitted; and Fig. 3 shows a piezo-electric wave filter employed in a lowerimpedance circuit, as is obtained in the case of an intermediate frequency stage of certain radio receiving systems.

Referring to Fig. 1 of the drawings, a loop antenna LA is shown, connected to a variable condenser K, these being tuned so as -to' receive the radio frequencies composed in a band of radio signals as well as the corre-- sponding carrier frequency. Thej impressed signals are transmitted through a wave filter including piezo-electric elements, the details of which are to be described hereinafter, and are impressed upon the grid and filament electrodes of a three-element vacuum tube V, which may be employed to act as a demodulator. The filament of this vacuum tube is maintained at a suit-able electron-emitting temperature by means of a battery 13,. The grid electrode is connected to the filament through a resistance R and a battery B whereby a suitable negativepotential is maintained at fthe grid electrode with respect to the filament. The plate electrode of the vacuum tube .V is connected to the-filament through the primary winding of'the transformer T and a battery B the battery B providing the required operating positive potential for the plate electrode. The'secondary winding of thetransformer T couples I the output circuit of the vacuum tube V with a wired circuit M. It will be apparent that while the transformer T is represented to interconnect the output circuit of the vacuum tube 'V with the wired circuit M, any other well known coupling arrangement may be einployed'instead thereof, preferably one including a pair of suitable reactance elements;

The electrical wave filter interconnecting the loop antenna circuit and the grid and filament electrodes of the vacuum tube V, may be composed of identical or substantially identical sections each including suitably selected series elements and suitably selected shunt elements. Thus, in Fig. l of the drawings, two sections of a plurality are shown, each including a piezo-electric crystal PE as the series element and an anti-resonant circuit including winding L and a condenser C as the shunt element. tals PE are preferably identical or substantially identical in their structure and char acteristics, and accordingly, vibrate at the same or at substantially the same frequencies.

If a piezo-electric crystal such as the one The piezo-electric crysemployed herein and designated as PE were connected to a source of alternating current,

thelfrequency of which was adjustable, the

pieZo-electric crystal would exhibit a capacis tative'reactance below some frequency such as f the magnitude of thefcapacitative reactance decreasing as the frequency f isapproached. Beyond the-frequency f the. re-

actance would become inductive rather gradually, and as another frequency f is approached, theinductive reactance would decrease very suddenly, becoming a capaci tative reactance atand beyond the frequency 72. The change from a high inductive reactance to a high capacitative reactance usually takes place within a narrow range of the frequency spectrum. Thereafter,- the capacitative reactance decreases in magnitude with increase in frequency. It will be understood that anyparticular.piezo-electric device may undergo these changes, ice, from capacitative reactance to "inductive reactance, and then again from inductive reactance to capacitative reactance, at a number of different frequencies within-the spectriun. In the arrangement shown in Fig. 1 of the drawings,

the piezo-electric devices- PEare selected so that these changes in the character of the reactances are substantially the same and take place at substantially the same frequencies.

Byassigning suitable values to the ele V ments employed in the various sections of the electrical wave filter shown interconnecting the loop antenna circuit and the grid and filament electrodes of the vacuum tubeV, at system can be provided which will trans mit sinusoidal currents having frequencies lying within a preassigned range, which may include, for example, a carrierfrequency to which the range is related, while at thesa me time effectively suppressingthe transmission of currents lying outside'ofthe preassigned range as well as the coresponding carrier frequency. 1 V j Fig. 2, of the drawings shows a radio receiv-j ing system for receiving any one ofaplurality of bands of signals and the corresponding filament electrodes of vacuum tube carrier frequency. A receiving antenna RA is shownconnected to groundthrough a wind-.

. ,Winding w, is coupled, to winding W1.

Winding W is connected-to'the grid and V through a novel form of electrical Twave filter, including piezo-electric elements, this wave filterbeing onewhich is capable of adjustment in order that one or another of a plurality ofbands composed of high frequency components may become freely trans mitted.

- The-electrical wave :filter shown in of the drawings may be composed of identical or substantially identical sections, each of which may include a group-of piezoeelectric elements and a group of corresponding reactance elements. Here two groups of pieZo-electric crystals andthree groups-of reactance elementsare shown, yet it will be understood that this arrangement may-include a third group of piezo-electric crystals and that it may include, in addition,,still other groups of piezo-electric crystalsand corresponding groups of reactance elements. Each group of crystals may be graded.- in accordance with frequencies. Thus, the na-v turalvibratory periodsof the crystals PE PE' and vPE may be arrangedin ascend-- ing or descending order. Asrhas already been stated, the piezoelectric crystal in one section are the same or substantially the; same as the-piezo-electric crystals in each .of the other sections. Thus, the natural vibras tory periods of the crystals designated PE may be the same or substantially the same, and the natural vibratory periods 10f the. crystals designated PE and those designated Q W be the e or substantiallyv the" same, respectively;

A' switch S may be associated wit h'the crystalsof-ieach section so as toconnect one oranother of theseinto circuit. Thus, switch S may be manipulated soas to interconnect the various crystals designated as PE When soconnected thesecrystals are set into vibration, their operation being in synchronism- Similarly, when the piezoelectric crystals PE 'or PE are connected into cir.

cuit, they are also rendered vibratory, oper atinfg in synchronism at difi'erent'frequencies, respectively The manipulation ofthe switches designated as S serves to. intercon- Eect the series elements of the electrical wave lter. v

The shunt elements of the disclosed filter may each be composed of aninductance L 7 in- 'parallel relationshipwith-any one of a plurality of'condensers designated in the drawings asv G G and Q switch S is. present in each shunt elementin order to selectively connect any one of the condensers inparallel relationship with the winding L.

The various. condensers maycorrespond to the piezo-electric devices representing. each group. Thus,'-condenser C may be of a value designed to function properly? whencrystals PE become interconnected, condensers C andCg being designed to operate in association with piezo-electric devices PEg and- P151 respectivel B manipulation of I the I I y V fit;

switches S in thevarious sections, the shunt elements may be brought into operation, each exhibiting anti-resonance at a frequency of predetermined value corresponding to the particular piezo-electric devices interconnected as series elements. Fig. 3 represents another embodiment o the invention in which one of the bands of signals composed of radio frequencies, as well as the corresponding carrier frequency, may be selected by manipulation of a condenser K connected across the winding W An oscillator O is shown transmitting current of adefinite frequency to be employed in beating the selected band, as well as the corresponding carrier frequency, to produce a band of intermediate frequencies and a cor responding carrier frequency considerably lowered in the frequency spectrum. The oscillator O is terminated in a resistance Z, the resistance Z and battery B being con nected in series with winding lVZ between the gridand filament electrodes of the vacuum tube V. It will be understood that the oscillator 0 may be any arrangement for producing sustained oscillations, preferably one of the vacuum tube type. Current of the oscillator O is 'beaten'with the selected band of si nals in the input circuit of the vacuum tu e V acting as a modulator to produce a band of intermediate frequencies and a correspondingly lowered carrier frequency. These are transmitted through an amplifier A and are substantially impressed upon an electrical wave filter which preferably includes pieZo-electric elements. Devices PE represent two of a plurality of series elements, while winding L and condenser C are interconnected in shunt relationship to provide an anti-resonant circuit and these represent one of a plurality of shunt elements. This electrical wave filter of thepiezo-electric type freely transmits to the circuit M through transformer T the intermediate band of frequencies and the corresponding carrier frequency. Apparatus may be connected to the circuit M to permit the beating of the intermediate'band of frequencies and the corresponding carrier frequency in order that the original signals may become reproduced.

It will be understood that suitable impedance elements may be added to the filter structure or structures shown herein to bring about any desirable transmission characteristics therefor. the values of these additional elements being determined from convenience" of design or may be made to satisfy some specified requirement such as, for example, that the system shall have a definite and predetermined impedance at some certain frequency or at some certain range or band of frequencies.

It will be further understood that while. the filter structure or structures shown in the drawings may be of the band pass class, any desired electrical wave filter for any range of frequencies or for any circuit network may be designed with suitably selected combina' tions of piezo-electric devices, within the scope of this invention.

It will be apparent that the principles of this invention are clearly suitable to provide an electrical wave filter composed of groups of piezoelectric elements whereby certain selected piezo-electric elements may become interconnected and rendered vibratory in order that any predetermined band of frequencies may become freely transmitted thereby.

It will also be understood that the number of sections required for a particular wave filter will depend upon the degree to which it is desired to extinguish currents to be filtered out. Ifthe number of sections is increased, the ratio of the magnitude of the current of any particular frequency'entering the filter to the magnitude of the current of that frequency leaving the filter may be greatly increased.

While this invention has been pointed out in certain particular arrangement merely for the purpose of illustration, it will be understood that the general principles of this inventionmay be applied to other and widely varied organizations without departing from the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. In a signaling system, the combination of means for receiving a plurality of bands of frequencies, a piezo-electric wave filter having a network ofvcontiguous'rccurrent sections, and means to adjust said filter to selectively and freely transmit any desired band of frequencies while greatly suppressin the frequencies within all other bands.

2. In a signaling system, the combination of a circuit upon which are impressed bands of modulated signals, an electrical wave filter including a plurality of piezo-electric devices,'certain of which may be selected and interconnected in a definite relationship, and means whereby only one band of modulated signals may become transmitted through said electrical wave filter very freely while all other bands become greatly suppressed.

3. In areceiving system for receiving any one of-a plurality of bands of frequencies each lying within definite limits of the spectrum, including a plurality of sets of piezoelectric devices, the various sets being equally graded at substantially the same frequencies, one pieZo-electric device of one set corresponding to a similar piezo-electric device in each of the other sets, the method of changing the receptivity from one band of frequencies to another band, which consists in selecting the'same piezo-electric device in the various sets and transmitting currents of frequencies lying within a predetermined -b and,

transmitting the currents of the band to be current sections each having a plurality of the selected devices being brought into vibratlon.

4. In a receiving system for receiving any one of a plurality of bands of frequencies each lying within definitelimits of the spec'- trum, the system including a plurality of sets of p-ieZo-electric devices graded equally and with substantially the same frequencies, a

piezo-electric device of one set corresponding to a similar piezo-electric device in-each of the other sets, the method of changing the receptivity" from oneband of frequencies to another, which consists in selecting and in terconnecting in definite relationship certain similar piezo-electric devices of the various sets corresponding to the band to be received,

received, andvibrating the selected and in terconnected piezo-electricdevicesl 5. The combination of a plurality of repiezo-electric devices graded in accordance with frequencies, the piezo-electric devices" in one recurrent section being substantially the same as the plezo-electric devices in every other recurrent section, an anti-resonant cir- Y cuit the natural period of which may be cuits for saidvacuum tube, said input circuit including a plurality of recurrent sec-- tions, each recurrent section including'a plurality of piezo-electric devices of different natural frequencies and a plurality of reactances one corresponding to each piezo-electrio device, and means for selectinga desired piezo-electric device in each recurrent section tothe exclusion of all other piezo-electric devices in that section, the plurality of piezoelectric devices in one section having natural frequencies which are substantially the same as the plurality of piezo-electric devices each of the other sections.

7. A receiving system comprising a three element vacuum tube, input and output circuits for said vacuum tube, said input circuit including a plurality of recurrent sections, each section including a plurality of'piezoelectric devices and a plurality of corresponding reactive elements, and means in each of the various sections for selecting and Qperatively associating any. desired piezoelectric device and the tive element. V 8. The combination of a plurality of groups of piezo-electric devices, the piezoelectric devices of one group having natural frequencies different from each other and the same natural frequencies respectively asthe piezo-electric devices in each of the other corresponding reac- 7 groups, and means for selecting and vibrating a particularpiezo-electricdevice in each .of the various groups, the selected piezonism. r QQThe combination of a plurality of groups of "pie'zo-electric devices, the piezolectric devices being ;vibrated in synchroelectric devices in'onegroup having natural frequencies which are different from each other and which are respectively equal tothe natural frequencies of the piezo-electric devices in any other group, means for selecting a particular pie'zo-electric device in eachof the groups having the same naturalfrequency, and means forvibrating the selected piezo-electric devices in synchronism.

10. The combination of a plurality be groups of piezo-electric devices, those piezoelectric devices in anyone group having subi stantially different natural frequencies, the

natural frequencies-of the piezo-electric devices in one group being substantially the same as the natural frequenciesof the piezoelectric devices in any other group, a plural- M .ityof reactive elements one corresponding to each different natural frequency, avacuum tube system, andmeans for selecting and associating thepiezo-electric devices of the various groups corresponding to-a particular 1 natural frequency and the corresponding reactive element with'said vacuum tube system;

11'. An electrical wave filter comprising a plurality of recurrent sections, eachsection including a plurality of piezo-electric devices and a plurality of corresponding reactive elements, and means for selecting the desired piezo-electr'ic devicein each section and thecorresponding reactive ele'ment'and inter connecting the selected piezo-electric devices.

12. An electrical-system including a plurality of sections, section having a plu-i rahty of plezo-electric elements and a plurality of corresponding reactive elements,

and means for selecting any desired piezoelectric element and the corresponding reactive element in each of the various sections,

the selected piezo-electric devices being interconnected in series relationship, the sein shunt relationship. 7

13. In an electrical wave'filter, the comlected reactive elements being interconnected bination of a plurality of groups of piezoelectric elements, the piezo-electric elements 5 of each group having. substantially different frequency characteristics, the frequency characteristics of the piezo-electric devices of one group being substantially the same as the frequency characteristics of the piezoelectric devices in anyother group, and means for selecting and interconnecting a; particular-piezo-electric element in each of the various groups and vibrating the selected piezo-electric elements in synchronism.

14. ThG'COmbHHttIOII of a radio frequency circuit, a vacuum tube system, and an electrical Wave filter including a plurality of contiguous recurrent sections arranged to freely transmit any desired band of radio 5 signals from the radio frequency circuit to the vaccum tube system, each section of said electrical Wave filter including a piezo-electrio device.

15. The combination of a radio frequency M circuit, a vacuum tube system, an electrical Wave filter including plurality of similar contiguous recurrent sections which are interconnected and couple the ra io frequency circuit With the vacuum tube system, each section of said electrical Wave filter including a piezoelectric device, and means to adjust the electrical ave filter to permit the free transmission of any one of a plurality of bands of radio signals which may be impressed on the 20 radio frequency circuit to said Vacuum tube system, to the exclusion of all signals having frequencies lying outside the selected band.

16. The combination 01 a Wave collecting device capable of collecting radio frequency 2. signals, a vacuum tube system, a piezo-electric structure coupling the Wave collecting device with the vacuum tube system, said piezo-electric structure including a plurality of uous recurrent sections each or": 3 Which comprises a piece-electric device, tl e various piezo-electric devices being interconnected in tandem, and means whereby the piezo-electric structure may freely transmit only'those currents of radio frequencies lying- Within any one of a plurality of bands each occupying a predetermined portion oi the frequency spectrum to the exclusion of currents of all other frequencies.

In testimony whereof, I have signed my #9 name to this specification this 28th day of December 1927.

CHARLES H. FETTER. 

